Ozone is a powerful disinfectant and is used to oxidize organic contaminants from drinking water, including the naturally occurring taste- and odor-causing compounds. Ozone is also used in the effluent from the secondary treatment of wastewater to remove trace organic contaminants and endocrine disrupting compounds (EDCs) from the water before reuse as indirect potable water or discharge to a water body. EDCs may be found in, for example, pharmaceutical residues, pesticides and herbicides, and many ingredients of personal care products. This application of ozone for the tertiary treatment of wastewater is rising faster than the other uses of ozone due to water scarcity and concerns over the impact of EDCs on aquatic life. For most applications involving treatment of drinking water or municipal wastewater, before disposal to streams and lakes the typical ozone dose is in the range of 3 mg of ozone per liter of water to 5 mg/L. The water flow rate in these applications depends on the population it serves and is usually about 700 m3/h or greater. The ozone dose for treatment of industrial waste water and for specialized applications such as color removal range from 10 mg/L to several hundred mg/L but the water flow rate is lower at around 150 m3/h, or less.
For the synthetic organic contaminants such as MTBE, TCE, 1,4 dioxane, etc. which are typically found in chemical-contaminated ground water sites, an “advanced oxidation” process is used for the water treatment. The advanced oxidation process combines hydrogen peroxide with ozone dissolved in water to produce highly-reactive hydroxyl radicals which oxidize the recalcitrant organic contaminants. Hydroxyl radicals are produced by the reaction between ozone and hydrogen peroxide or a catalyst in the aqueous phase. The advanced oxidation process is used to treat industrial process water for reuse in the plant, or before discharge of the contaminated effluent to municipal sewers or the environment.
Ozone gas is commonly produced in a corona discharge-based generator from air or high-purity oxygen. The typical concentration of ozone in gas phase ranges from 3 to 14% depending on the generator power and concentration of oxygen in the gas feed used for ozone generation. Ozone-based water treatment processes depend upon transfer of ozone from the gas phase to the water phase for oxidation of organic contaminants. Various processes have been used to transfer ozone from gas phase to liquid phase for the purposes of water treatment.
All of these processes depend on creating mixing or relative motion at the gas-liquid interface, and the ozone solubility at the process-operating conditions to transfer ozone from gas phase to liquid phase. Key criteria for selection of equipment include the energy consumed during operation, the desired ozone dose rate, the cost and size of the equipment, and the ozone mass-transfer efficiency. The ozone mass-transfer efficiency is defined as the percentage of the ozone gas that is introduced during the process that is dissolved in the aqueous phase. To increase the amount of ozone transferred and thus the ozone mass-transfer efficiency, it is desirable to use high-concentration ozone of approximately 8% or higher (by weight) at an operating pressure higher than the atmospheric pressure. The high ozone concentration reduces the amount of gas that has to be handled in the gas-liquid mixing apparatus and increases the solubility of ozone in the water. Operating the process at higher than the atmospheric pressure also increases the solubility of ozone in water. The combined effect of the high ozone concentration and higher operating pressure is an increased gas mass-transfer driving force, which improves the ozone mass-transfer efficiency. The residual ozone in the effluent gas stream from the process represents the energy that is wasted in the ozone generator and results in increased oxygen costs. The effluent gas stream has to be passed through an ozone-destruct unit in order to remove the undissolved ozone before discharging the gas safely to the atmosphere.
Commercial ozone generators typically produce ozone at 15 psig to 30 psig (103.4 kPa to 206.8 kPa) gas pressure. At higher pressures, the energy efficiency and generating capacity of corona discharge generators is reduced, making it more expensive to produce ozone at pressures greater than 15 psig (103.4 kPa). This limits the economical ozone gas source to 15 psig (103.4 kPa). To take advantage of the better mass-transfer efficiency at high gas pressure, a venturi eductor-based process is favored over other methods that operate at or slightly above the atmospheric pressure. However, the venturi eductor by itself provides a low dissolution of ozone from gas phase to water. The mass transfer occurs mostly in the throat section where the gas is aspirated by the motive water stream. Downstream of the eductor, mixing is inefficient and provides only limited additional mass transfer. To improve ozone dissolution downstream of the eductor, static mixers are used. The static mixer requires high fluid velocities passing through it in order to mix gas into water for ozone transfer. The minimum velocity required is specific to the static mixer and results in a large pressure drop across the mixer, thus increasing the energy requirements for the process.
One known process for the transfer of ozone from gas phase to liquid phase for the purposes of water treatment is a bubble column or basin reactor, which comprises a large column or basin and gas diffusers located at the bottom of the column or basin. In some embodiments, the gas diffusers may be located under approximately 15 to 20 feet (4.6 to 6.1 meters) of water. The column or basin is continuously filled with contaminated water and ozone gas is introduced through the gas diffusers. Fine bubbles of ozone gas rise through the water in the column or basin, which provides mixing and turbulence of the water in the basin and promotes dissolution of the ozone into the water (also referred to herein as “ozone transfer”). Ozone transfer efficiency can be improved by capturing and recirculating undissolved ozone from the top of the column or basin and/or passing the ozone through a series of columns or basins using baffles. Depending on the ozone dose and the basin design, in some embodiments one or two sections of the basin are used for gas sparging and the remaining sections are used to achieve the dissolved gas removal and the desired ozone contact time (CT) for the water. The treated water is removed from the basin after the desired CT has been reached.
The basin contactor process operates within a narrow range of total gas-flow rates in order to achieve good mixing and mass transfer in the basin. If the total gas flow rate is reduced, the gas bubbles rise through the water column without significant mixing or turbulence in the water. This reduces the ozone mass-transfer efficiency in the contactor. As ozone gas generation is the major cost of the process, the reduced ozone mass-transfer efficiency in the contactor makes the process less economical. The lack of proper mixing due to reduced gas flow also leads to non-uniform distribution of ozone in the basin, and could reduce the CT below that required by the relevant water-disinfection regulations. To overcome these problems, the total gas flow rate is maintained constant by lowering the ozone concentration in the gas. A typical ozone treatment process uses high purity oxygen as the feed gas to generate ozone. When the ozone concentration in the feed gas is reduced in response to low ozone demand, either due to a low water-flow rate or a lower contaminant concentration, a greater amount of high-purity oxygen is required per unit mass of ozone to maintain the constant total flow of gas through the bubble diffusers. The use of a large fraction of high-purity oxygen increases the unit cost of treating water and results in a waste of energy during periods of low ozone demand.
Another drawback to the use of a basin contactor is that the fine pores in the gas diffuser (often micron size) clog over time, and thereby significantly affect the performance of the contactor. Clogging of the diffusers requires that they be cleaned or replaced periodically, leading to process downtime and increased maintenance costs. The treatment of wastewater is also a greater challenge for the traditional fine-bubble diffuser-based basin contactor method as the wastewater carries a higher concentration of the fine suspended solids in it that lead to frequent clogging of the diffuser and poor performance in the contactor. Water flow rates in both drinking water and wastewater treatment plants vary significantly during the day and over the seasons, which makes operation of most traditional contacting methods expensive during low ozone-demand periods. Other disadvantages of the diffuser-based process include that: large, deep basins are required for effective transfer of ozone to water, thus increasing costs and space requirements; channeling of gas bubbles reduces the efficiency of mass transfer; and the process is not amenable to high-pressure operation, which would increase the dissolution of the ozone gas in water.
Another known ozone transfer method is the use of a venturi ejector, in which water flows through the venturi and ozone gas is educted at the throat of the venturi. This venturi-based method can only be used effectively in systems with relatively low water flow rates. In systems that operate at relatively large flow rates, a portion of the water can be diverted into a “slip stream” on which the venturi is located. The slip stream is then injected back into the main stream and mixed into the main stream by turbulent flow. The diverted stream venturi method is typically only effective for relatively low-dose ozone transfer (e.g., 10 mg/L or less). This method also requires high cross-flow velocity of the influent water in the main pipe to provide mixing of two-phase flow from the jet into the main flow and to carry the mixed stream a longer distance in the pipe than in a slow-moving water flow. The purpose of the high-velocity jet is to achieve additional ozone transfer in the main flow through rapid dissipation of the turbulent energy of the jets. The high velocity of the two-phase jet required for effective mixing and mass transfer of ozone in the main flow leads to a high pressure drop across the injection nozzles. The high pressure drop represents energy that is wasted to achieve the ozone transfer and is supplied by the side stream pump. The energy requirement for this method is typically much greater than that for the basin contactor.
In another variation of venturi-based ozone transfer, static mixers can be used downstream from the educator or gas-injection nozzle to achieve additional mixing and dissolution of ozone in the water phase. The system is simpler to design as it has no moving parts. But the mixing and gas dispersion for good ozone transfer through a static mixer requires a highly-turbulent flow of gas and liquid. This requires high gas-liquid velocity through the static mixer which results in a higher pressure drop across it than any other process of gas dissolution. The minimum gas-liquid velocity required is specific to the static mixer used and the process can only be operated in a narrow range of water and gas flow rates to achieve the turbulent flow needed for ozone dissolution. The efficiency of the ozone transfer suffers dramatically when the water flow rate is reduced below the optimum operating range for the static mixer. This is a huge challenge for the plant operations because in drinking water and wastewater plants the water-flow rate changes considerably with demand during the day and over the seasons.
There have been attempts to perform ozone transfer using turbine contactors, which operate by aspirating gas through hollow turbine shafts and agitators. Turbine contactors do not appear to be well-suited to ozone transfer applications for several reasons. As compared to the ozone transfer methods described above, turbine contactors have relatively high power requirements. In addition, the ratio of ozone gas to water entering the turbine contactor must be kept relatively constant for efficient operation, which limits the ability to adjust ozone dosing. Turbine contactors are not well-suited for catalytic ozonation because the powdered catalyst will plug the channels through which the ozone gas is aspirated.
Packed columns are rarely used for ozone transfer into liquid phase because this type of reactor has very low ozone mass-transfer efficiency, and therefore a very tall column is required to achieve typical ozone dosing. Packed columns also have low void volume, which limits the water flow rate through a given diameter column. Packed columns can be used for fixed bed catalytic reactions with ozone but, due to low mass-transfer efficiency of ozone, are expensive to build and operate.
Impinging jets have also been used to enhance mixing between gas and liquid phases in ozone-transfer systems. In such systems, a high-velocity jet of two phase flow is impacted with another two-phase flow jet or with a stationary surface. In these impinging-jet processes, two jets of gas-liquid stream are impacted at high velocity from opposite sides. A portion of the treated water may be recycled and mixed with the influent contaminated water, and then fed through a pump to form the jets. In addition, undissolved ozone may be captured downstream in a phase separator and recycled through the jets. Impinging jets can be used as the sole mixing reactor, or can be used in combination with other mixing reactors. The design and operation of an ozone transfer system including impinging jets is complex due to the need for precision location of the impact zones. In addition, the jets have high power requirements and the range of flow rates that can be accommodated by this type of system is limited. Accordingly, this method is rarely used for large-volume water treatment applications, for example drinking- and wastewater-treatment applications.
Accordingly, there is a need for an improved method of ozone transfer that overcomes the deficiencies of the methods of the prior art.